1. Field of the Invention
The present invention relates to a condensing optical device for use in an optical pickup for performing recording and/or playback of an information signal with respect to a different three types of optical discs, an optical pickup, and an optical disc apparatus using the optical pickup.
2. Description of the Related Art
In recent years, an optical disc capable of high-density recording (hereinafter, referred to as “high-density recording optical disc” in which recording/playback of a signal is performed by using an optical beam with a wavelength of about 405 nm by a blue-violet semiconductor laser has been proposed as a next-generation optical disc format. An example of the high-density recording optical disc has been proposed in which the thickness of a cover layer for protecting a signal recording layer is made thin, for example, to 0.1 mm.
When providing an optical pickup compatible with the high-density recording optical discs, an optical pickup having compatibility with optical discs having different formats, such as a CD (Compact Disc) with a use wavelength of about 785 nm, a DVD (Digital Versatile Disc) with a wavelength of about 655 nm, and the like, according to the related art has been desired. Thus, there has been demand for an optical pickup and an optical disc apparatus having compatibility between optical discs having formats in which the disc structure and consequently laser specifications thereof differ.
A method is known which realizes recording or playback of an information signal with respect to three types of optical discs having different formats. In this case, two types of objective lenses and two types of optical systems for DVD/CD and high-density recording optical disc such as shown in FIG. 43 are provided, and the objective lenses are switched for each use wavelength.
An optical pickup 130 shown in FIG. 43 realizes recording and/or playback of different types of optical discs by providing two types of objective lenses 134 and 135. The optical pickup 130 includes a light source unit 132, such as a laser diode or the like, which has an emission unit for emitting an optical beam with a wavelength of about 785 nm with respect to an optical disc, such as a CD or the like, and an emission unit for emitting an optical beam with a wavelength of about 655 nm with respect to an optical disc, such as a DVD or the like, a light source unit 131, such as a laser diode or the like, which has an emission unit for emitting an optical beam with a wavelength of about 405 nm with respect to a high-density recording optical disc, an objective lens 134 for an optical disc, such as a DVD, a CD, or the like, and an objective lens 135 for a high-density recording optical disc. The optical pickup also includes collimator lenses 142A and 142B, quarter-wave plates 143A and 143B, redirecting mirrors 144A and 144B, beam splitters 136 and 137, gratings 139 and 140, a photosensor 145, a multi lens 146, and the like.
An optical beam with a wavelength of about 785 nm emitted from the light source unit 132 transmits the beam splitter 136 and the beam splitter 137, and is input to the objective lens 134. The optical beam is condensed on a signal recording surface of an optical disc having a protection layer (cover layer) with a thickness of 1.1 mm by the objective lens 134.
Similarly, an optical beam with a wavelength of about 655 nm emitted from the light source unit 132 is input to the objective lens 134 through the same optical path, and is condensed on a signal recording surface of an optical disc having a protection layer with a thickness of 0.6 mm. Return light with a wavelength of 785 nm and return light with a wavelength of 655 nm reflected at the signal recording surfaces of the optical discs are detected by the photosensor 145 including a photodetector or the like through the beam splitter 137.
An optical beam with a wavelength of about 405 nm emitted from the light source unit 131 is reflected at the beam splitter 136, and is input to the objective lens 135 through the beam splitter 137. The optical beam is condensed on a signal recording surface of an optical disc having a protection layer with a thickness of about 0.1 mm by the objective lens 135. Return light with a wavelength of 405 nm reflected at the signal recording surface of the optical disc is detected by the photosensor 145 through the beam splitter 137.
With the optical pickup shown in FIG. 43, two types of objective lenses of the objective lens 134 for DVD/CD and the objective lens 135 for high-density recording optical disc are provided, thereby realizing recording and/or playback of different three types of optical discs, that is, realizing compatibility between multiple types of optical discs.
However, the optical pickup such as described above has the following problems. First, each optical disc has a different optimum inclination of objective lens, and in the above-described optical pickup, the use of the two objective lenses 135 and 134 causes a situation where the attachment angle of the actuator of the objective lenses 135 and 134 to lens holders may be unsuitable. Accordingly, the optimum inclination of objective lens cannot be realized with respect to an optical disc. As a result, the quality of a playback signal may be degraded. In the above-described optical pickup, since the two types of objective lenses 135 and 134 are used, it is necessary to provide two types of optical systems, such as redirecting mirrors, collimator lenses, quarter-wave plates, and the like, resulting in an increase in the number of parts. For this reason, costs may be increased, and the optical pickup may be increased in size. Further, in the above-described optical pickup, the two objective lenses 135 and 134 need to be mounted on an objective lens driving actuator, resulting in an increase in the weight of the actuator and degradation of sensitivity.
Meanwhile, an optical pickup is studied in which the above-described problems are solved, and optical parts are further simplified by using a single objective lens in common with respect to multiple types of optical discs and three types of use wavelengths. A basic principle for providing an objective lens corresponding to optical beams with three types of wavelengths is to provide a diffraction unit, such as a diffraction optical element or the like, in the optical path upstream of the objective lens, thereby inputting the optical beam to the objective lens in the state of diffusive/convergent light and correcting spherical aberration due to the combination of use wavelength and media.
However, in the known optical pickup being studied, the configuration has involved diffraction units, which are provided on multiple surfaces, a diffractive surface needs to have a spherical surface shape different from the spherical surface of the objective lens, or a liquid crystal element having a complex configuration needs to be provided in the optical path upstream of the objective lens. In each of these configurations, the lens units, the diffraction units, the liquid crystal elements, or the like are individually formed and then assembled. For this reason, a rather high level of precision is necessary for positioning the units and attaching multiple diffraction surfaces, leading to more and increasingly troublesome and complicated steps in manufacturing, and problems of failure to meet the necessary precision.
For example, as described in JP-A-2004-265573, an optical pickup is proposed in which a diffraction unit is provided on one surface, but this has only realized compatibility of two wavelengths. In order to realize compatibility of three wavelengths, there is a need to separately provide an objective lens corresponding to another wavelength, resulting in an increase in the number of optical parts and complication of the configuration (see JP-A-2004-265573). Further, when an optical pickup having compatibility of three wavelengths is provided, there is also a need to increase light use efficiency or to reduce unwanted light incidence.